When I was in school studying NAND devices (2004-2010) we were quite apprehensive at the long term quantum stability of 4-layer devices.

This (the past 20 years of improvement) is an incredible feat of engineering.

This is 321 physical layers of silicon in an IC, not 321 charge levels.

QLC flash - with 16 charge levels, for four bits per cell - is pretty common nowadays, but that's as far as it goes so far. And stability is indeed a concern; modern flash devices rely heavily on error correction.